"Rock snot," a.k.a. Didymosphenia geminata is a fast-spreading single-cell algae that is invading the once pristine streams popular with fly fisherman.
Didymo has a natural tendency to grow upstream in fast-moving rivers and creeks, but it can spread by clinging to fishing equipment, especially the felt-bottom waders that fly fishermen use to keep from slipping on river bottoms.
Didymo is considered native to parts of North America, where it was found in higher elevations with cold, nutrient-poor waters. But in the last 20 years, the single-celled diatom seems to have morphed into a more aggressive invasive species, spreading from British Columbia across the continent to New York.
Unlike other algae, which float on the surface, Didymo clings to rocks on the bottom of rivers, streams and lakes. At times it grows furiously in blooms that can cover a river bottom from bank to bank, smothering the stone flies, worms and other organisms that trout and other sport fish live on.